Trans-bis(alkynyl) ruthenium complexes: synthesis, structure and reactivity

This Thesis describes the synthesis and characterisation of a series of trans-bis(alkynyl) ruthenium complexes, trans-[Ru(C≡CR)2(L)4], to better understand how the variation of the metal ancillary ligands (L) affect electronic structure and spectroscopic properties, chemical reactivity, and behaviour in metal|molecule|metal junctions. Reactions of cis-[RuCl2(dppm)2] with terminal alkyne HC≡CC6H4-4-R, in the presence of TlBF4 and base, gives into trans-bis(alkynyl) complexes, trans-[Ru(C≡CC6H4-4-R)2(dppm)2], for electron withdrawing R groups or cationic η3-butenynyl complexes, E-­[Ru(η3-{HC(C6H4-4-R)=CC≡C(C6H4-4-R)})(dppm)2]+ for electron donating R groups. Reactions of cis-[RuCl2(dppm)2] with di-terminal alkynes HC≡CC6H4-2,5-X2-4-C≡CH, in the presence of TlBF4 and [NnBu4]Cl, gives trans-[RuCl(C≡CC6H2-2,5-X2-4-CCl=CH2)(dppm)2], inferring a quinoidal cumulene intermediate. Multi-metallic trans-bis(alkynyl) {Ru(dppe)2} complexes, varying in binding groups and bridging ligands, have been prepared. Reversible oxidation processes, whilst corresponding to the number of integrated metal centres, exhibit a high degree of alkynyl character in all cases. The vibrational and electronic spectra of both neutral and oxidised complexes are complicated by the presence of numerous spectroscopically distinct rotamer conformations and redox isomers. For example in the case of mono-oxidised complexes, a principal low-energy (π-π*) NIR band is exhibited along with multiple higher energy (MLCT-type) NIR bands, which can be assigned by comparison with smaller model systems. Finally, trans-bis(alkynyl) {Ru{P(OEt)3}4} complexes have been synthesised. As a result of the increased (pseudo D4h) molecular symmetry and consequent fewer distinct rotamer conformations, a lesser number of NIR bands are exhibited for trans­[Ru(C≡CR)2{P(OEt)3}4]+ than bis-chelating dppm and dppe derivatives. Between trans-[Ru(C≡CR)2(PPʹ)] (PPʹ = (dppe)2, {P(OEt)3}4) complexes, the {Ru(dppe)2} derivatives give rise to conductance histograms with additional features. These features are attributed to contacts formed at or across the dppe-phenyl rings, leading to suggestions that phosphite complexes might be novel ‘insulated’ molecular wires